(CNN) -- An oil sheen about four miles long has appeared in the Gulf of Mexico near the site of the worst oil spill in U.S. history, a Coast Guard spokesman said Thursday.

It was not immediately clear where the oil is coming from, said Petty Officer 3rd Class Ryan Tippets.

The Coast Guard found out about the oil sheen on September 16 after someone spotted it on a satellite image from the multinational oil and gas company BP, Tippets said. A Coast Guard response team went to the location to collect samples, and sent them to the Coast Guard Marine Safety Lab in Connecticut for testing. Test results are expected in a few weeks, Tippets said.

The service's Marine Safety Unit Morgan City, in Louisiana, is heading up the investigation of the spill, which is in the part of the Gulf officially designated MC252. No one has reported any adverse effects on the environment or marine life, Tippets said.

The sheen is near the spot where, on April 20, 2010, BP's Deepwater Horizon drilling rig exploded over the Macondo well, killing 11 workers and spewing oil that spread across a huge portion of the Gulf.